Aldose Reductase is an enzyme that converts sugars to sugar alcohols. Their accumulation in lens tissue results in osmotic imbalance and diabetic cataract formation and nerve tissue diabetic neuropathy. In other tissues its role is less known. We believe that a knowledge of the chemistry of aldose reductase will be valuable and necessary for a rational approach to regulation of this enzyme and slowing the pathology associated with diabetes. In recent years we have purified this enzyme to homogeneity from a variety of mammalian and yeast sources. Using the pure enzyme we have carried out extensive rate studies as well as other physical, chemical and biological studies of the whole molecule. We are proposing to learn the exact chemical sequence of the amino acids or the primary structure at the active site of the enzyme molecule. We believe these approaches and study of the structure and regulation of this enzyme will lead to a more precise understanding and control of these pathological effects observed in diabetes, and be useful to all researchers in this field.